Nonlinear Cardiac Dynamics and Morning Dip: An Unsound Circadian Rhythm

ABSTRACT

OBJECTIVES: We studied the circadian rhythm of nonlinear heart rate dynamics in healthy subjects. BACKGROUND: The frequency of sudden cardiac death increases in the morning. The relationship between decreased complexity of heart rate dynamics and sudden cardiac death has been documented. An understanding of the circadian variation in the complexity of cardiac dynamics may be important and to predict and prevent this sudden cardiac death. METHODS: Dynamic 24-hour electrocardiographic recordings were obtained from 30 healthy ambulant subjects aged 41 to 50 years and the digitized data was partitioned into sections of 30 minutes' duration. For each section, four indexes obtained from separate algorithms of nonlinear dynamics of RR interval - correlation dimension, Lyapunov exponent, approximate entropy, and fractal dimension - were calculated. Normalized low- (0.04-0.1 hertz) and high-frequency (>0.15 hertz) components were also calculated. RESULTS: All the four indexes of nonlinear dynamics showed a remarkably similar circadian rhythm a prominent morning dip preceded by a steep decline during the late night, a recovery during the evening and a peak around midnight. In the morning, the low frequency component rose rapidly with concomitant withdrawal of the high frequency component. CONCLUSION: The complexity of cardiac dynamics decreases significantly in the morning, and this may contribute to the ominously increased rate of cardiac death in the morning hours.